I wanted to set up half a dozen Hi-Brite LEDs under a ledge to provide illumination, and I had a convenient connection to a UK mains line (240v AC single phase).
The LEDs are 3v, 30ma. And size was an issue. So I bought this LED driver from ebay. https://www.ebay.co.uk/itm/Led-driver-power-supply-transformer-240v-dc-12v-adapter-1-36W-300ma-8-12w-5v/362673175057?ssPageName=STRK:MEBIDX:IT&var=631973633249&_trksid=p2057872.m2749.l2649
It fits nicely in the space available.
The spec says 3-9v output, but the heading mentioned 12v, so I rigged it up with a test LED and put a 680 ohm1/8W resistor in series - the LED calculation for 12v says 700. That worked nicely with around 2.8v across the diode, so I wired up the LED string - two parallel wires with 6 LEDs and Diodes across the wires, and connected the other end to the mains.
What happened was:
1 - the leds lit nicely
2 - the resistors got hot - about 100Cdeg.
3 - the voltage across the wires measured at about 40v with a digital multimeter
4 - the voltage across each diode measured about 25v with the same multimeter
5 - the driver got warm
I do not understand the voltage measurements. I was expecting around 5-10v across the parallel wires, and 2-3v across each LED. The LEDs have not blown, so the voltage MUST be within limits, but my meter suggests that it is not. I don't understand what is happening, and don't want to go to the next stage, which is putting in a PWM switch to dim the LEDs without knowing a bit more about what is going on. Has anyone got any ideas? Or suggestions for further measurement which would give more information?
DG
The LEDs are 3v, 30ma. And size was an issue. So I bought this LED driver from ebay. https://www.ebay.co.uk/itm/Led-driver-power-supply-transformer-240v-dc-12v-adapter-1-36W-300ma-8-12w-5v/362673175057?ssPageName=STRK:MEBIDX:IT&var=631973633249&_trksid=p2057872.m2749.l2649
It fits nicely in the space available.
The spec says 3-9v output, but the heading mentioned 12v, so I rigged it up with a test LED and put a 680 ohm1/8W resistor in series - the LED calculation for 12v says 700. That worked nicely with around 2.8v across the diode, so I wired up the LED string - two parallel wires with 6 LEDs and Diodes across the wires, and connected the other end to the mains.
What happened was:
1 - the leds lit nicely
2 - the resistors got hot - about 100Cdeg.
3 - the voltage across the wires measured at about 40v with a digital multimeter
4 - the voltage across each diode measured about 25v with the same multimeter
5 - the driver got warm
I do not understand the voltage measurements. I was expecting around 5-10v across the parallel wires, and 2-3v across each LED. The LEDs have not blown, so the voltage MUST be within limits, but my meter suggests that it is not. I don't understand what is happening, and don't want to go to the next stage, which is putting in a PWM switch to dim the LEDs without knowing a bit more about what is going on. Has anyone got any ideas? Or suggestions for further measurement which would give more information?
DG
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